The standard method for locating a driverless vehicle at a predetermined location is by means of a locating pin such as that illustrated in FIG. 15 of U.S. Pat. No. 3,858,626. A locating pin of that type has an accuracy of about 0.015 inches. In an automated assembly line system, there is a need for substantially greater accuracy. A more accurate system for locating a driverless vehicle at a predetermined location, with an accuracy of .+-.0.005 inches in three planes, is described in U.S. Pat. No. 4,428,300. In that system, a shoe or pad having a tapered nose is mounted on the underside of the vehicle and cooperates with a cluster of at least three rollers so as to elevate the vehicle so that support wheels thereon are spaced from the tracks while centering the vehicle in a direction transverse to the direction of travel of the vehicle. One of the rollers is employed to elevate the vehicle. The remaining two rollers are centering rollers which center the vehicle as it is being elevated. Only one of the centering rollers may contact a tapered surface of the shoe as the vehicle is being centered resulting in non-uniform roller wear and possible loss of centering accuracy. The rollers must be precisely spaced so as to contact opposite non-tapered sides of the shoe once the vehicle has been centered in order to retain the vehicle in the centered position as shown in FIG. 3 of the patent. The width of the shoe must therefore be maintained in strict tolerance. If the width of the shoe is out of tolerance and too narrow, there will be excessive clearance between the shoe and the centering rollers thereby preventing the vehicle from being precisely centered. If the width of the shoe is too great, the shoe will be jammed between the rollers such that the vehicle cannot be moved to or from the centered position.
The problem solved by the present invention is that of precisely locating a vehicle which may have a length of 10-15 feet with an accuracy of .+-.0.005 inches or better in three planes with a minimum number of rollers without scrubbing or scuffing of the centering rollers and while insuring that the centering rollers experience the same, uniform wear so that there is no play during centering regardless of wear.